There will probably always be empty seats as most of the standing tickets are standing/unreserved seating. I would have thought they can't physically sell too many tickets because what if no one chose to sit in the seats and you ended up with a crush on the pitch. I think you can only sell enough tickets so that it's safe if everyone chose to stand and hope that enough people sit on the seats to make it more comfortable for people who do choose to stand. There are also people who go on these corporate VIP things who aren't really fans and spend half the show in the bar.